1. Field of the Invention
This invention relates to improvements in a CVD method and a CVD system accomplishing the method in order to deposit a silicon oxide film on a substrate under a reaction between an organic Si source and O.sub.3, the method and system being usable in the field of production of semiconductor devices.
2. Description of the Prior Art
CVD (Chemical Vapor Deposition) methods are well known and used to deposit a silicon oxide (SiO.sub.2) film on a substrate by using reaction between an organic Si source and ozone (O.sub.3). Of these CVD methods, an atmospheric pressure TEOS(tetra ethoxy silane)-O.sub.3 -CVD method been extensively used. The silicon oxide film obtained by this CVD method has a characteristic to be able to effectively fill the narrow spaces between Al wirings in a highly integrated LSI without forming voids, and therefore attention has been paid to the silicon oxide film as a future insulating film between layers. In this CVD method, polymerization reaction of TEOS and oxygen radical produced under decomposition of O.sub.3 is used in which oligomer as the polymer resulted from the polymerization reaction is different in its adsorption characteristics (to the substrate) depending upon the physical properties of the substrate on which the oligomer is to be deposited. For example, TEOS-O.sub.2 plasma silicon oxide film which is hydrophilic and often used as the substrate (film) for the silicon oxide film produced by the atmospheric pressure TEOS-O.sub.3 -CVD method; however, the absorption of the oligomer or deposit to the surface of such a hydrophilic film is degraded during the film deposition so that the properties of the silicon oxide film deposited on the substrate film is lowered. This has been known, for example, from a Japanese monthly technical magazine "Semiconductor World, 1992, January, Pages 140 to 153".
There are two methods for supplying gases in the atmospheric pressure TEOS-O.sub.3 -CVD method. One is a premixing method in which used gases (O.sub.3 and TEOS) are mixed with each other prior to supply to the surface of the substrate (film). This method requires a pretreatment such as a plasma treatment and an ethanol coating to the substrate (film) in order to improve a substrate dependency (dependency of adsorption characteristics of the deposit to the characteristics of the substrate) as seen from the graph of FIG. 1 indicating an etching rate (mm/min). In FIG. 1, (A) represents a case of film deposition under the pretreatment of ethanol coating onto a plasma TEOS; (B) a case of film deposition under the pretreatment of N.sub.2 plasma onto a plasma TEOS; (C) a case of film deposition made on a plasma TEOS; and (D) a case of film deposition made on a bare Si substrate. The etching rate was measured upon the etching with buffered hydrofluoric acid. It will be understood that such a pretreatment unavoidably increases the number of production steps and production equipment cost.
Another method is a postmixing method in which mixing of used gases is made immediately above the surface of the substrate (film). More specifically, in this method, the used gases (O.sub.3 and TEOS) are separated from each other by a curtain of inert gas such as N.sub.2 and subsequently mixed with each other immediately above the surface of the substrate (film). It has been confirmed that the substrate dependency is improved by the postmixing method as compared with the premixing method.
A conventional plasma CVD system accomplishing the postmixing method is schematically illustrated in FIG. 2. In FIG. 2, a wafer W' is put on a conveyer belt 12' and moves from the left-hand side to the right-hand side or in a direction indicated by an arrow in the figure and will pass through a location under a gas injector 1'. The gas injector 1' supplies mixed gases onto the surface of the wafer W', so that a silicon oxide film is deposited on the wafer W'. This gas injector 1' is provided with five gas injector nozzles or slits. A slit 2' for TEOS is formed at the central portion of the gas injector 1'. Two slits 3A', 3B' for N.sub.2 are formed on the opposite sides of the slit 2'. Two slits 4A', 4B' are respectively formed outside the slits 3A', 3B'
N.sub.2 gas supplied through a N.sub.2 gas line 9' is subjected to a flow rate control by a mass flow controller 6' and supplied to a bubbler 5' in which TEOS is stored. TEOS is in a liquid state at ordinary temperature and bubbled at a predetermined temperature under the action of N.sub.2, and then ejected from the slit 2' together with N.sub.2. N.sub.2 gas to be ejected from the slits 3A', 3B' is supplied through a N.sub.2 gas line 10' and subjected to a flow rate control of a N.sub.2 mass flow controller 7'. O.sub.3 gas to be ejected from the slits 4A', 4B' is supplied through a O.sub.3 /O.sub.2 gas line 11' and is subjected to a flow rate control under the action of a O.sub.3 /O.sub.2 mass flow controller 8'. TEOS and O.sub.3 ejected from the slits are separated from each other by N.sub.2 (serving as a curtain) ejected from the slits 3A, 3B, so that TEOS and O.sub.3 cannot be mixed before coming near the surface of the wafer W'. This is intended to avoid reaction between TEOS and O.sub.3 in gas phase thereby to suppress generation of particle as much as possible.
However, difficulties have been encountered in the above-discussed plasma CVD system using the postmixing method, in which the polymerization reaction of TEOS under the action of oxygen radical resulting from decomposition of O.sub.3 becomes difficult thereby degrading the properties (for example, the etching rate) of the resultant silicon oxide film. This is guessed to be occur owing to lowering in contact probability between oxygen radical and TEOS molecular thus making an insufficient polymerization reaction.
As a countermeasure for the above difficulties, it may be proposed to lower the flow rate of N.sub.2 gas ejected from the slits 3A, 3B under the action of the mass flow controller 7 or to make the flow rate zero. This makes the postmixing method approach the premixing method, and therefore it seems that the polymerization of TEOS proceeds to improve the film properties and quality of silicon oxide. However, there arise problems in which the substrate dependency is strengthened owing to the oligomer produced by the polymerization reaction, so that the film properties are lowered particularly on a hydrophilic surface of the substrate.